This invention relates to the use of current liming elements and positive temperature coefficient resistivity (PTC) elements in circuit breakers to limit the arcing and interruption pressure that results from the operation of a circuit breaker under short circuit conditions.
Circuit breakers are widely used in residential and industrial applications for the interruption of electrical current in power lines upon conditions of severe overcurrent caused by short circuits or ground faults. One of the problems associated with the process of interruption of current during severe overcurrent conditions is arcing. Arcing occurs between the contacts of circuit breakers used to interrupt the current and is highly undesirable for several reasons. Arcing causes deterioration of the circuit breaker contacts and produces gas pressure within the circuit breaker. Arcing also necessitates circuit breakers have a larger separation between the contacts in the open position to extinguish the arc during high current faults. Prior art devices have used a number of approaches to limit the occurrence of arcing. For example, in heavy duty switchgear, the circuit breaker contacts may be enclosed in a vacuum or in an atmosphere of SF6. Both of these approaches are expensive.
Another approach to limit the amount of arcing is the use of a resistor connected in parallel with the contacts of the circuit breaker. Upon opening of the contacts, current can flow through the shunt resistor, effectively reducing the amount of arcing in the contacts. The current flowing through the resistor is less than the short circuit current that would flow through the contacts in the absence of the resistor.
A current limiting circuit breaker or current limiter typically can provide limitation to the let-through current during a short circuit. The current limiter can interrupt a short circuit before the available current reaches zero. In other words, the current limiter can dramatically reduce both the peak current (Ip) and the let-through energy (I2t) values compared to conventional circuit breakers. In conventional current limiting breakers, almost 100% of the interruption energy goes to generate arc and pressure upon a short circuit. In an attempt to address this problem and to achieve the above current limitation functions, costly components are being added to conventional circuit breakers.
The present invention provides for a cost efficient manner to increase current limitation effectiveness and decrease the interruption pressure within the circuit breaker, thereby improving the interruption rating of the circuit breaker and greatly reducing the potential damage to end-use equipment. Therefore, this invention allows for the design of better performing and less expensive current limiters than conventional current limiting circuit breakers.
The present invention provides a current limiting circuit breaker having a plurality of current responsive devices for opening a pair of contacts upon short circuit conditions. One such device is a conventional magnetic tripping mechanism. The other device utilizes an insulating object driven by a magnetic force caused by the short circuit current. Upon opening of the contacts with the use of the insulating object, let-through current flows through a secondary contact, positioned on the insulating object, to a positive temperature coefficient resistivity element which limits the current and arcing in the contacts. In an alternative embodiment, at least one steel component is added to increase the magnetic force, thereby providing a greater force on the insulating object. An insulation component is also added to further suppress any arc generated between the contacts when going from a closed state to an open state. In another alternative embodiment, the magnetic tripping mechanism is actuated by the device utilizing the insulating object driven by the magnetic force.
Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.